Apologies if this has been reported before or you no longer fix bugs in BI...

Confirmed in 2.63.5.

Load default settings, open a new file and duplicate (Sh-D) the default cube. Duplicate the material of the new cube, so they have different materials. Select the Lamp and in Object properties add it to a new group. In the material settings of one of the cubes, choose the group name in the Light Group settings. Disable the renderability of the lamp. Render.

You will see that the cube that does not have its material restricted to the light group will be lit, while the other won't, even though the lamp isn't supposed to render.

File attached, demonstrating this.